1. Field of the Invention
The present invention relates to a magneto-optical recording medium allowing high-density recording and reproduction of information.
2. Description of the Related Art
A magneto-optical disk is known as a high-density recording medium, and an increase in its recording density is demanded with an increase in quantity of information. Increasing the recording density of the medium can be realized by shortening the space between adjacent recording marks. However, the reproduction of each recording mark is limited by the size of a light beam (beam spot) on the medium. In the case that the density of the recording marks is set so that only one recording mark is present inside the beam spot, an output waveform corresponding to xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d can be observed as a reproduced signal according to whether or not the recording mark is present inside the beam spot.
However, in the case that the density of the recording marks is increased so that a plurality of recording marks are present inside the beam spot, the reproduced output does not change irrespective of movement of the beam spot on the medium, so that the output waveform becomes linear and the presence or absence of recording marks cannot be distinguished. The reproduction of such small recording marks having a period shorter than the size of the beam spot may be effected by reducing the size of the beam spot. However, the size of the beam spot is limited by the wavelength xcex of light output from a light source and the numerical aperture NA of an objective lens, so that the spot size cannot be sufficiently reduced.
Recently commercially available is a magneto-optical disk drive adopting a reproducing method using a magnetically induced super-resolution (MSR) technique for reproducing a recording mark smaller than the size of the beam spot by the use of an existing optical system. The MSR is a reproducing method such that while one mark present inside the beam spot is being reproduced, another mark is masked to thereby increase a reproductive resolution. Accordingly, such an MSR disk medium requires at least a mask layer or reproducing layer for masking the other mark so that the one mark is reproduced during signal reproduction, in addition to a recording layer for recording marks.
A magneto-optical recording medium using a perpendicularly magnetized film as the reproducing layer is proposed in Japanese Patent Laid-open No. 3-88156, for example. In the prior art described in this publication, however, an initial magnetic field of several kilooersteds is required for initialization of the reproducing layer. Accordingly, the disk drive using this magneto-optical recording medium cannot be reduced in size. A magneto-optical recording medium using a magnetic film having an axis of easy magnetization in a plane at a room temperature and an axis of easy magnetization in a perpendicular direction at a predetermined temperature or higher as the reproducing layer is proposed in Japanese Patent Laid-open No. 5-81717, for example.
Further, an MSR medium having a reproducing layer, a recording layer, and an intermediate layer interposed between the reproducing layer and the recording layer is described in U.S. Pat. No. 6,020,079. In the MSR medium described in this U.S. Patent, a reproducing laser beam is directed onto the medium to form a temperature distribution composed of a low-temperature region, an intermediate-temperature region, and a high-temperature region inside the beam spot. The low-temperature region and the high-temperature region form a double mask, and a recorded mark is read from only the intermediate-temperature region. Since the double mask is formed by the low-temperature region and the high-temperature region, the intermediate-temperature region for reading a recorded mark can be greatly reduced in size, thereby allowing high-density recording and reproduction.
An MSR magneto-optical recording medium commercially available at present is either of a land recording type such that data is recorded on lands only or of a groove recording type such that data is recorded on grooves only. Attention has recently been given to a land/groove recording type such that data is recorded on both lands and grooves, so as to achieve higher-density recording and reproduction. In a magneto-optical recording medium adopting this land/groove recording type, a transparent substrate having a plurality of lands and a plurality of grooves alternately formed is used.
In the case that a magneto-optical recording film capable of performing MSR reproduction is formed on this substrate, there is a problem that a bias magnetic field required for erasure of data becomes large. This is considered to be due to the fact that the center distance between adjacent land and groove, i.e., the track pitch, is small, for example. When the track pitch is further reduced, a larger reproducing magnetic field is required in general. This tendency is remarkable especially in the reproduction at the grooves. The conventional medium has a tendency that when the composition of the medium, the film deposition conditions, etc. are selected so as to reduce the erasing magnetic field, the reproducing magnetic field becomes large in general.
To provide such a large erasing magnetic field and/or reproducing magnetic field, a large magnet must be installed in the magneto-optical disk drive, so that it is difficult to reduce the size of the magneto-optical disk drive. Further, a power consumption in the magneto-optical disk drive also becomes large. Also in the normal magneto-optical recording medium for recording data on only the lands or only the grooves, it is important to reduce the erasing magnetic field, thereby reducing the size of the magnet in the magneto-optical disk drive and reducing the power consumption in the magneto-optical disk drive.
It is therefore an object of the present invention to provide a magneto-optical recording medium which can reduce the erasing magnetic field.
It is another object of the present invention to provide a magneto-optical recording medium which can reduce not only the erasing magnetic field, but also the reproducing magnetic field.
It is a further object of the present invention to provide a land/groove recording type magneto-optical recording medium which can suppress an increase in the erasing magnetic field with a decrease in the track pitch.
In accordance with an aspect of the present invention, there is provided a magneto-optical recording medium comprising a magnetic reproducing layer having an axis of easy magnetization in a direction perpendicular to its layer surface; a magnetic intermediate layer formed on said magnetic reproducing layer and having an axis of easy magnetization in a plane at a room temperature; and a magnetic recording layer formed on said magnetic intermediate layer and having an axis of easy magnetization in a direction perpendicular to its layer surface; said magnetic reproducing layer having a composition of GdxFeCoy where 22 at %xe2x89xa6xxe2x89xa625 at % and 16 at %xe2x89xa6yxe2x89xa623 at %.
Preferably, said magnetic reproducing layer is formed from a rare earth-transition metal amorphous alloy film, and is transition metal-dominant such that the magnetic moment of transition metal is dominating over the magnetic moment of rare earth. Further, said magnetic intermediate layer has a Curie temperature of 200xc2x0 C. or less, and a temperature at which the magnetization in said magnetic intermediate layer in the case of a single layer changes to perpendicular magnetization is 130xc2x0 C. or more. More preferably, the magnetic intermediate layer is deposited by sputtering at a sputter rate of 7 nm/sec to 9 nm/sec under an Ar gas pressure of 2 Pa to 4 Pa.
In accordance with another aspect of the present invention, there is provided a magneto-optical recording medium comprising a GdFeCo reproducing layer having an axis of easy magnetization in a direction perpendicular to its layer surface, said reproducing layer having a thickness of 40 to 57 nm; a GdFeCoSi intermediate layer formed on said reproducing layer and having an axis of easy magnetization in a plane at a room temperature, said intermediate layer having a thickness of 38 to 50 nm; and a TbFeCo recording layer formed on said intermediate layer and having an axis of easy magnetization in a direction perpendicular to its layer surface.
Preferably, said reproducing layer has a composition of GdxFeCoy where 22 at %xe2x89xa6xxe2x89xa625 at % and 16 at %xe2x89xa6yxe2x89xa623 at %. Further, said reproducing layer has a thickness of 45 to 50 nm, and said intermediate layer has a thickness of 43 to 48 nm.
In accordance with a further aspect of the present invention, there is provided a magneto-optical recording medium for land and groove recording, comprising a transparent substrate having a plurality of lands and a plurality of grooves alternately formed; a magnetic reproducing layer formed on said transparent substrate and having an axis of easy magnetization in a direction perpendicular to its layer surface; a magnetic intermediate layer formed on said magnetic reproducing layer and having an axis of easy magnetization in a plane at a room temperature; and a magnetic recording layer formed on said magnetic intermediate layer and having an axis of easy magnetization in a direction perpendicular to its layer surface; said magnetic reproducing layer having a composition of GdxFeCoy where 22 at %xe2x89xa6xxe2x89xa625 at % and 16 at %xe2x89xa6yxe2x89xa623 at %.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.